Concerter and a method for controlling a converter

ABSTRACT

The invention relates to a converter provided with a resonant circuit ( 16 ), which comprises a control device ( 24 ) adapted, in connection with a commutation of the phase current (i ph ) from a semiconductor element of turn-off type of a first current valve ( 2, 3 ) to a rectifying member of a second current valve ( 3, 2 ) to effectuate a turn-on of the auxiliary valve ( 18 ) of the resonant circuit with a varaible time delay (t d ) after the turn-off of the first current valve, the converter comprising a member ( 35 ) for determining said time delay in dependence on the magnitude and the direction of the phase current (i ph ) and the magnitude and the direction of a determined unbalance so that the unbalance is corrected at least partly when the control device applies the time delay (t d ). The invention also relates to a method for controlling such a converter.

FIELD OF THE INVENTION AND PRIOR ART

[0001] The present invention relates to a converter according to thepreamble of claim 1 and a method for controlling such a converter.

[0002] The invention particularly relates to a VSC-converter. AVSC-converter for connection between a direct voltage network and analternating voltage network is previously known e.g. from the thesis“PWM and control of two and three level high voltage source converters”by Anders Lindberg, Royal Institute of Technology, Stockholm, 1995, inwhich publication a plant for transmitting electric power through adirect voltage network for high-voltage direct current (HVDC), whileutilizing such converters, is described. Before the creation of thisthesis, plants for transmitting electric power between a direct voltagenetwork and an alternating voltage network have been based upon the useof network commutated CSC (Current Source Converter)—converters instations for power transmission. However, in this thesis a totally newconcept is described, which is based on instead using VSC (VoltageSource Converter)—converters for forced commutation for transmittingelectric power between a direct voltage network being voltage stifftherethrough, in the case in question for high-voltage direct current,and alternating voltage networks connected thereto, which offers severalconsiderable advantages as compared to the use of network commutatedCSC-converters in HVDC, among which it may be mentioned that theconsumption of active and reactive power may be controlled independentlyof each other and that there is no risk of commutation faults in theconverters and thereby no risk of commutation faults being transmittedbetween different. HVDC-links, as may occur with network commutatedCSC:s. Furthermore, it is possible to feed a weak alternating voltagenetwork or a network without any generation of its own (a deadalternating voltage network). There are also further advantages.

[0003] The inventive converter may be included in a plant fortransmitting electric power through a direct voltage network forhigh-voltage direct current (HVDC), in order to e.g. transmit theelectric power from the direct voltage network to an alternating voltagenetwork. In this case, the converter has its direct voltage sideconnected to the direct voltage network and its alternating voltage sideconnected to the alternating voltage network. The inventive convertermay however also be directly connected to a load, such as a high-voltagegenerator or motor, in which case the converter has either its directvoltage side or its alternating voltage side connected to thegenerator/motor. The invention is not limited to these applications; onthe contrary, the converter may just as well be used for conversion in aSVC (Static Var Compensator) or a back-to-back-station. The voltages onthe direct voltage side of the converter are with advantage high, 10-400kV, preferably 130-400 kV. The inventive converter may also be includedin other types of FACTS-devices (FACTS=Flexible Alternating CurrentTransmission) than the ones mentioned above.

[0004] In order to limit the turn-off losses in the semiconductorelements of turn-off type of the current valves of the converter, i.e.the losses in the semiconductor elements of turn-off type when these areturned off, it is previously known to arrange capacitive members in theform of so-called snubber capacitors connected in parallel across therespective semiconductor element of turn-off type. It is also known toprovide the converter with a so-called resonant circuit for rechargingsaid snubber capacitors in connection with commutation of the phasecurrent. Hereby, it will also be possible to limit the turn-on losses inthe semi-conductor elements of turn-off type of the current valves, i.e.the losses in the semiconductor elements of turn-off type when these areturned on.

[0005] A type of converter provided with a resonant circuit that hasbeen developed and come into use is the so-called ARCP-converter(ARCP=Auxiliary Resonant Commutated Pole). An ARCP-converter comprises aseries connection of at least two intermediate link capacitors arrangedbetween the two poles of the direct voltage side of the converter, saidseries connection being divided into two equal parts through a midpoint,denominated intermediate link midpoint in the following, so as toprovide in the intermediate link midpoint a voltage essentiallycorresponding to the mean value of the voltage between the two poles. Inconnection with a commutation of the phase current assisted by theresonant circuit, a current pulse corresponding to a certain charge willflow in the resonant circuit either into or out of the intermediate linkmidpoint. These current pulses tend to displace the voltage in theintermediate link midpoint in one or the other direction. However, thephase currents are sinusoidal in most cases, the sum thereof thereforebeing zero. In theory, the total sum of the charge supplied to orwithdrawn from the intermediate link point through the resonant circuitduring the commutations in the different phase legs of the converter istherefore also to become zero over a fundamental-tone period, i.e. themean value in time of the difference between the voltage across one ofthe intermediate link capacitors and the voltage across the otherintermediate link capacitor is in theory to remain zero during afundamental-tone period. However, this mean value will in practice bedisplaced in one or the other direction due to for instance unbalance inthe phase current, defects in the components of the converter or in thecontrol of the converter. This displacement normally constitutes a slowprocess taking place over several fundamental-tone periods. The voltageunbalance caused by said displacement may for instance result in thatthe voltage across the current valve that is to be turned on during acommutation process never reaches the value zero, which in its turnresults in increased turn-on losses and in worst case in destruction ofthe semiconductor element of turn-off type of said current valve, sincethe current valve will be turned on when it has voltage across itself.In HVDC-applications, where the intermediate link midpoint of theconverters is grounded, said voltage unbalance may result in undesiredground currents between the intermediate link midpoints of convertersthat are connected to each other, which ground currents i.a. entailincreased losses.

[0006] In this description and the subsequent claims the expression“unbalance of the voltage in the intermediate link” refers to adifference between the voltage between one of the poles and theintermediate link midpoint and the voltage between the intermediate linkmidpoint and the other pole. In the ideal case, this difference is zero,i.e. the voltage is in the ideal case the same across the twointermediate link capacitors. The “direction of the unbalance” refers towhether the unbalance implies that the voltage between said one of thepoles and the intermediate link midpoint is larger or smaller than thevoltage between the intermediate link midpoint and the other pole, i.e.whether the unbalance depends on that an additional charge amount hasbeen supplied to or withdrawn from the intermediate link midpoint. The“magnitude of the unbalance” refers to the magnitude, i.e. the absolutevalue, of the difference between the voltage between one of the polesand the intermediate link midpoint and the voltage between theintermediate link midpoint and the other pole.

[0007] In previously known ARCP-converters the resonant circuit isactivated in connection with communication of the phase current from asemiconductor element of turn-off type of a first current valve to arectifying member of a second current valve, i.e. in connection with theturning-off of a semiconductor element of the first current valve, whenthe phase current is so low that the switching time for the voltage inthe phase output otherwise would be unreasonably long.

[0008] A method for correcting occurring voltage unbalances in theintermediate link of an ARCP-converter is described in U.S. Pat. No.5,880,949. This method is based on that the outgoing current valve isturned off with a variable time delay after the activation of theresonant circuit. Consequently, an additional current pulse is conductedthrough the resonant circuit in connection with a commutation assistedby the resonant circuit so as to thereby supply an additional chargeamount to or withdraw an additional charge amount from the intermediatelink midpoint, which additional charge amount corrects the unbalance atleast partly. Consequently, this method is based on that an additionalcurrent of suitable magnitude is added to the resonant current in caseof a noted unbalance of the voltage in the intermediate link. Adisadvantage with this method is that said additional current causes anincrease of the peak value of the current through the resonant circuit,and consequently increased losses in the resonant circuit. Theseincreased losses can be considered to be in direct contravention of thepurpose of the resonant circuit, which purpose is to reduce the lossesin the converter in connection with commutation of the phase current.

OBJECT OF THE INVENTION

[0009] The object of the present invention is to make it possible tocorrect occurring unbalances of the voltage in the intermediate link ofa converter, while avoiding the above-mentioned increase of losses inthe resonant circuit.

SUMMARY OF THE INVENTION

[0010] According to the invention, said object is achieved by means of aconverter according to claim 1 and a method according to claim 9.

[0011] The inventive solution implies that the control device of theconverter, in connection with a commutation of the phase current from asemiconductor element of turn-off type of a first current valve to arectifying member of a second current valve, is made to effectuate aturn-on of the auxiliary valve with a variable time delay after theturn-off of the first current valve, said time delay being determined independence on the magnitude and the direction of the phase current andthe magnitude and the direction of a determined unbalance so that theunbalance is corrected at least partly when the control device appliesthe time delay. Consequently, the unbalance is corrected in that theauxiliary valve in connection with a commutation of the phase current isturned on with a certain time delay after the turn-off of the outgoingcurrent valve, in contrast to the method known from U.S. Pat. No.5,880,949 where the outgoing current valve is turned off with a certaintime delay after the turn-on of the auxiliary valve. Consequently,according to the present invention, no adding of an additional currentto the resonant current is used for the correction of the unbalance, andan increase of the peak value of the current through the resonantcircuit, which increase is unfavourable with respect to the losses, isthereby avoided.

[0012] According to a preferred embodiment of the invention, the valueof the unbalance of the voltage in the intermediate link is filtered,the time delay being determined in dependence on this filtered value.Hereby, it is secured that the corrections essentially will relate tothe slow and undesired displacements of the voltage in the intermediatelink that are remaining during several periods of the network voltage.

[0013] Further preferred embodiments of the inventive converter and theinventive method will appear from the dependent claims and thesubsequent description.

BRIEF DESCRIPTION OF THE DRAWING

[0014] The invention will in the following be more closely described bymeans of embodiment examples, with reference to the appended drawing. Itis shown in:

[0015]FIG. 1 a simplified circuit diagram illustrating a converteraccording to a first embodiment of the invention,

[0016]FIG. 2 a simplified circuit diagram illustrating a converteraccording to an alternative embodiment of the invention,

[0017]FIGS. 3-5 current curves and voltage curves during differentcommutation processes,

[0018]FIG. 6 a simplified block diagram illustrating a control systemfor effectuation of the inventive method,

[0019]FIG. 7 a diagram illustrating the change of the current throughthe resonant circuit and the phase voltage during a commutation processwith a timedelayed and a non-timedelayed, respectively, turn-on of theauxiliary valve of the converter, and

[0020]FIG. 8 a diagram illustrating a displacement of the limit valuefor positive phase currents.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] A converter according to an embodiment of the invention isillustrated in FIG. 1. The converter is here a so-called VSC-converter.In FIG. 1, only the part of the converter that is connected to one phaseof an alternating voltage phase line is shown, the number of phasesnormally being three, but this may also constitute the entire converterwhen this is connected to a single phase alternating voltage network.The shown part of the converter constitutes a so-called phase leg, and aconverter adapted for instance to a three-phase alternating voltagenetwork comprises three phase legs of the type shown.

[0022] VSC-converters are known in several designs. In all designs, aVSC-converter comprises a number of so-called current valves, each ofwhich comprising a semiconductor element of turn-off type, such as anIGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-OffThyristor), and a rectifying member in the form of a diode, a so-calledfree wheeling diode, connected in anti-parallel therewith. Eachsemiconductor element of turn-off type is normally in high-voltageapplications built up of several series connected, simultaneouslycontrolled semi-conductor components of turn-off type, such as severalseparate IGBT:s or GTO:s. In high-voltage applications a comparativelyhigh number of such semiconductor components is required in order tohold the voltage to be held by each current valve in the blocking state.In the corresponding manner, each rectifying member is built up ofseveral series connected rectifying components. The semiconductorcomponents of turn-off type and the rectifying components are in thecurrent valve arranged in several series connected circuits, each ofwhich circuits comprising i.a. a semiconductor component of turn-offtype and a rectifying component connected in anti-parallel therewith.

[0023] The phase leg of the converter illustrated in FIG. 1 has twocurrent valves 2, 3 connected in series between the two poles 4, 5 of adirect voltage side of the converter. A direct voltage intermediate link6 comprising two so-called intermediate link capacitors is arrangedbetween the two poles 4, 5. In the converter illustrated in FIG. 1 theintermediate link 6 comprises two series contacted intermediate linkcapacitors 7, 8. A midpoint 9, here denominated intermediate linkmidpoint, between these capacitors 7, 8 is here, as customary, connectedto ground, so as to provide the potentials +U_(d)/2 and −U_(d)/2,respectively, at the respective pole, U_(d) being the voltage betweenthe two poles 4, 5. The grounding point 9 may however be excluded, forinstance in SVC-applications.

[0024] A midpoint 10 of the series connection between the two currentvalves 2 and 3, which constitutes the phase output of the converter, isconnected to an alternating voltage phase line 11. In this manner, saidseries connection is divided into two equal parts with a current valve 2and 3, respectively, in each such part. In the embodiment with threephase legs, the converter consequently comprises three phase outputs,which are connected to a respective alternating voltage phase line of athree-phase alternating voltage network. The phase outputs are normallyconnected to the alternating voltage network via electric equipment inthe form of breakers, transformers etc.

[0025] In the embodiment shown, the respective current valve 2, 3comprises, in accordance with the above indicated, several seriesconnected circuits 12, each of which circuits comprising a semiconductorcomponent 13 of turn-off type, such as an IGBT, an IGCT, a MOSFET, aJFET, a MCT or a GTO, and a rectifying component 14 in the form of adiode, a so-called free wheeling diode, connected in anti-paralleltherewith. In the embodiment shown in FIG. 1, each current valve 2, 3comprises two series connected circuits 12 of the type described above,but the series connected circuits 12 may be larger as well as smaller innumber. Depending i.a. on the voltage for which the converter isdesigned, the number of said series connected circuits 12 in therespective current valve 2, 3 may extend from two up to several hundred.Each of the series connected circuits 12 of the respective current valve2, 3 is provided with a capacitor 15, here denominated snubbercapacitor, connected in parallel with the semi-conductor component 13 ofturn-off type included in the circuit. The capacitance of the respectivesnubber capacitor 15 must be so high that a good voltage distributionbetween the semi-conductor components 13 of turn-off type included inthe respective current valve is made possible in connection withturn-off of the semiconductor components of turn-off type of a currentvalve. The choice of capacitance of the snubber capacitors 15 is adaptedfrom case to case and depends i.a. on the current-handling capacity ofthe semiconductor components 13 of turn-off type and the rectifyingcomponents 14. The snubber capacitors 15 help to limit the turn-offlosses of the current valves, i.e. the losses in the semiconductorcomponents of turn-off type when these are turned off.

[0026] When the semiconductor components 13 of a current valve areturned off, the snubber capacitors 15 that are connected across thesesemiconductor components 13 will be charged. If the snubber capacitors15 keep this charge when the semi-conductor components 13 subsequentlyare turned on, turn-on losses will ensue in the semiconductor components13. The relatively high capacity snubber capacitors 15 that will comeinto question in this connection will in this case cause very highturn-on losses, which turn-on losses make the use of high switchingfrequencies impossible. In order to eliminate or at least reduce theseturn-on losses, and make possible the use of high switching frequencies,the snubber capacitors 15 are included in a resonant circuit 16. Hereby,it will be possible to accomplish discharge of the snubber capacitors 15of a current valve when the semi-conductor components 13 of the currentvalve are to be turned on, so that the voltage across the respectivesemiconductor component 13 is equal to or close to zero when it isturned on, whereby the turn-on losses are limited.

[0027] It is also possible to include a capacitor arranged between thephase output 10 and the midpoint 9 of the direct voltage intermediatelink in the resonant circuit 16.

[0028] The converters illustrated in FIGS. 1 and 2 are of the typedenominated ARCP-converter. The resonant circuit 16 is here of so-calledquasi-resonant type, which implies that the resonance only is initiatedwhen the current is to be commutated between two current valves, i.e.when the voltage on the phase output is to be changed-over.

[0029] In the embodiment shown in FIG. 1, the resonant circuit 16comprises a series connection of an inductor 17 and an auxiliary valve18 arranged between the phase output 10 and the midpoint 9 of saidseries connection of intermediate link capacitors 7, 8. The auxiliaryvalve 18 here comprises a set of two series connected auxiliary valvecircuits 19, each of which comprising a semiconductor component 20 ofturn-off type, such as an IGBT, an IGCT, a MOSFET, a JFET, a MCT or aGTO, and a rectifying component 21 in the form of a diode connected inanti-parallel therewith. The semiconductor components 20 of turn-offtype of the two auxiliary valve circuits 19 are arranged in oppositepolarity in relation to each other. This auxiliary valve 18 constitutesa bi-directional valve that can be made to conduct in one or the otherdirection.

[0030] In this description and the subsequent claims, the expressionauxiliary valve refers to a current valve included in the resonantcircuit 16 of the converter.

[0031] The auxiliary valve 18 may also comprise several series connectedsets of auxiliary valve circuits if considered appropriate, asillustrated in FIG. 2. In the embodiment illustrated in FIG. 2, theresonant circuit comprises an auxiliary valve 18 comprising severalseries connected sets 22 of auxiliary valve circuits, each set comprisestwo series connected auxiliary valve circuits 19 of the type describedabove. Only two series connected sets 22 of auxiliary valve circuits ofthe auxiliary valve 18 are shown in FIG. 2, but the number of such setsmay be considerably larger than that. The number of sets of auxiliaryvalve circuits in the auxiliary valve 18 may be optimised independentlyof the number of series connected circuits 12 in the current valves 2,3, and depends i.a. on the voltage the auxiliary valve is to be able tohold in the blocking state and the characteristics of the individualsemiconductor components 20 that are being used. Generally, it can beobserved that the auxiliary valve 18 in the blocking state only has tohold half the pole voltage, i.e. U_(d)/2, in contrast to the currentvalves 2, 3, which each has to be dimensioned so as to be able to holdthe entire pole voltage U_(d) in the blocking state.

[0032] Each set 22 of auxiliary valve circuits 19 in the auxiliary valve18 is suitably, as illustrated in FIG. 2, provided with its own controlunit 23, which is adapted to control the turn-on and turn-off of thesemiconductor components 20 of turn-off type included in the set, allcontrol units 23 of the auxiliary valve being connected to a commoncontrol device 24, which is adapted to send control signals to all thesecontrol units 23. Hereby, a simultaneous control of all the auxiliaryvalve circuits 19 of the auxiliary valve is secured.

[0033] It is further preferred that each of the semiconductor components13 of turn-off type included in the current valves 2, 3 of theconverter, as illustrated in FIG. 2, is provided with its own controlunit 25, which is adapted to control the turn-on and turn-off of thesemiconductor components 13, all control units 25 of the current valvesbeing connected to a common control device 24, which is adapted to sendcontrol signals to all the control units 25 included in a current valve2, 3. Hereby, a simultaneous control of all the semiconductor components13 of a current valve is secured. The control units 23 of the auxiliaryvalve and the control units 25 of the current valves are here connectedto one and the same control device 24, which is to be preferred.

[0034] There are three basic processes for commutation of the phasecurrent in a converter of the type illustrated in FIGS. 1 and 2, whichbasic processes will be briefly described in the following.

[0035] The current valve which is current carrying in the initial state,i.e. when the commutation process is initiated, is in this descriptionand the subsequent claims denominated “the first current valve” or “theoutgoing current valve”, and the current valve which is to be madecurrent carrying by the commutation is denominated “the second currentvalve”. It is realized that the one of the two current valves 2, 3illustrated in FIGS. 1 and 2 that at each specific commutation occasionconstitutes “the first” current valve and “the second” current valve,respectively, will vary from case to case.

[0036] In this description and in the subsequent claims, the expression“a commutation not assisted by the resonant circuit” implies that theseries connection of auxiliary valve 18 and inductor 17 included in theresonant circuit does not take part in the commutation process. However,the capacitive members, i.e. the snubber capacitors 15, will of coursetake part in this commutation process. In the corresponding manner, theexpression “a commutation assisted by the resonant circuit” implies thatthe series connection of auxiliary valve 18 and inductor 17 included inthe resonant circuit is taking part in the commutation process.

[0037] A first commutation process implies commutation of the phasecurrent from a semiconductor element of turn-off type of a currentcarrying first current valve 2, 3 to a rectifying member of a secondcurrent valve 3, 2 without any assistance of the resonant circuit 16.The commutation process is initiated in that the semi-conductor elementof turn-off type of the first current valve is turned off, whereupon thephase current i_(ph) produces a charging of the capacitive members ofthe first current valve, i.e. its snubber capacitors 15, and a dischargeof the capacitive members of the second current valve, i.e. its snubbercapacitors 15. The phase potential will hereby swing from one pole tothe other pole. In FIG. 3 the change of the current i_(lgbt) and thevoltage u_(lgbt) of the semiconductor element of turn-off type of thefirst current valve as well as the phase voltage u_(ph) during thecommutation process is illustrated. The semiconductor element ofturn-off type of the first current valve is turned off at the instantt₀, whereupon the current through the semiconductor element in the idealcase directly goes down to zero. In reality the semi-conductor elementwill have a certain reverse recovery current. The phase current i_(ph)will thereafter produce a charging of the capacitive members of thefirst current valve, the voltage there-across and thereby across thesemiconductor element of turn-off type increasing essentially linearlyfrom zero up to a value U_(d) corresponding to the voltage between thepoles 4, 5. It is realized that the turn-off of the semiconductorelement of the first current valve may take place essentially withoutany power dissipations.

[0038] A second commutation process implies commutation of the phasecurrent from a semiconductor element of turn-off type of a currentcarrying first current valve 2, 3 to a rectifying member of a secondcurrent valve 3, 2 with the assistance of the resonant circuit 16. Thiscommutation process is used at low phase currents in order to acceleratethe commutation. The commutation process is initiated by turning on thesemiconductor component of turn-off type (the embodiment according toFIG. 1), or whenever applicable the semiconductor components of turn-offtype (the embodiment according to FIG. 2), to which voltage is (are)applied in the auxiliary valve. At the same time as the semi-conductorcomponent(-s) of the auxiliary valve is (are) turned on, thesemiconductor element of the first current valve is turned off. Aresonance period will now follow, during which the resonant circuitprovides to the phase output 10 a current conducing to the charging ofthe snubber capacitors 15 of the first current valve and discharging ofthe snubber capacitors 15 of the second current valve. After the voltageacross the second current valve has fallen to zero or to a value closeto zero, the semi-conductor element of turn-off type of the secondcurrent valve is turned on. At the same time as or after thesemiconductor element of the second current valve is turned on, thesemiconductor component(-s) 20 of turn-off type that was (were)initially turned on in the auxiliary valve 18 is (are) turned off.

[0039]FIG. 4 illustrates the change of the current i_(res) through theresonant circuit and the phase voltage u_(ph) during the above-describedcommutation process. The semi-conductor component(-s) of turn-off typeof the auxiliary valve 18 is (are) turned on at the instant to. In thecase illustrated in FIG. 4, the semiconductor element of turn-off typeof the first current valve is turned off at the same instant t₀.

[0040] A third commutation process implies commutation of the phasecurrent from a rectifying member of a current carrying first currentvalve 2, 3 to a semiconductor element of turn-off type of a secondcurrent valve 3, 2 with the assistance of the resonant circuit 16. Thecommutation process is initiated by turning on the semiconductorcomponent of turn-off type (the embodiment according to FIG. 1), orwhenever applicable the semi-conductor components of turn-off type (theembodiment according to FIG. 2), to which voltage is (are) applied inthe auxiliary valve, whereupon a so-called ramp-up period is initiated.During the ramp-up period the current in the resonant circuit will,under the influence of the voltage u_(d1), u_(d2) across one of the twointermediate link capacitors 7, 8, increase from zero to a valuecorresponding to the phase current i_(ph). When the current through theresonant circuit reaches a value corresponding to the phase currenti_(ph), a resonance period begins, during which the snubber capacitors15 of the first current valve are being charged and the snubbercapacitors 15 of the second current valve are being discharged. When thevoltage across the second current valve has fallen to zero or to a valueclose to zero, the semi-conductor component of turn-off type of thesecond current valve is turned on. After the resonance period aso-called ramp-down period begins, during which the current in theresonant circuit decreases to zero from a value corresponding to thephase current i_(ph). When the ramp-down period is over and the currenthas fallen to zero in the resonant circuit, the semiconductorcomponent(-s) 20 of turn-off type that was (were) initially turned on inthe auxiliary valve 18 is (are) turned off.

[0041]FIG. 5 illustrates the change of the current i_(res) through theresonant circuit and the phase current u_(ph) during the above-describedcommutation process.

[0042] The inventive converter is preferably controlled withPWM-technique (PWM=Pulse Width Modulation), the control device 24 beingsupplied with signals representing the desired commutation instants froma modulator 30, schematically illustrated in FIG. 6.

[0043] The inventive converter is provided with means for measuring thephase current i_(ph), schematically indicated at 31 in FIG. 6, whichmeans is adapted to transmit measuring signals to the control device 24.

[0044] The previously described unbalance of the voltage in theintermediate link is i.a. caused by the current pulses flowing throughthe auxiliary valve 18 and the inductor 17 of the resonant circuit inconnection with commutations of the phase current, I.e. In connectionwith the above-mentioned second and third type of commutation process.The inventive converter comprises means for determining an unbalance ofthe voltage in the intermediate link. This means comprises a device 32for measuring of voltage and a processing unit 33 adapted to process themeasuring values from said device 32 for determining the magnitude andthe direction of said unbalance. The processing unit 33 may beintegrated in the control device 24, as illustrated in FIG. 6, orconstitute a unit being separate therefrom. According to a preferredembodiment of the invention, the device 32 comprises a first sensor 36for measuring the voltage u_(d1) between the intermediate link midpoint9 and one 4 of the poles and a second sensor 37 for measuring thevoltage u_(d2) between the intermediate link midpoint 9 and the otherpole 5, the processing unit 33 being adapted to calculate the differenceΔu_(d)=u_(d1)−ud₂ between said voltages in order to determine themagnitude and the direction of the voltage unbalance in the intermediatelink 6.

[0045] It is preferred that the means for determining an unbalance ofthe voltage in the intermediate link 6 comprises a device 34 forfiltering the value of said unbalance provided by the processing unit33. This device 34 may be integrated in the processing unit 33 or thecontrol device 24 or be separate therefrom.

[0046] According to the invention, the control device 24 is adapted, inconnection with a commutation of the phase current i_(ph) from asemiconductor element of turn-off type of a first current valve 2, 3 toa rectifying member of a second current valve 3, 2, to effectuate aturn-on of the auxiliary valve 18 with a variable time delay td afterthe turn-off of the first current valve 2. The converter comprises amember 35 for determining said time delay td in dependence on themagnitude and the direction of the phase current at the commutationmoment and the size and the magnitude of a determined unbalance so thatthe unbalance is corrected at least partly when the control deviceapplies the time delay t_(d). Said member 35 may be integrated in thecontrol device 24, as illustrated in FIG. 6, or be separate therefrom.

[0047] In a first extreme case the time delay t_(d)=0, in which case theauxiliary valve is turned on at the same time as the first, i.e. theoutgoing, current valve is turned off and the resonant circuit is fullyused in the commutation. In a second extreme case, the commutation takesplace completely without any assistance of the resonant circuit, i.e.the time delay td is so chosen that it will not flow any current throughthe inductor and the auxiliary valve of the resonant circuit during thecommutation. The latter case can be said to correspond to a case wheret_(d)=T/², where T stands for the duration of a commutation processtaking place without any assistance of the resonant circuit and is givenby the formula $T = \frac{C_{s} \cdot U_{d}}{i_{p\quad h}}$

[0048] where U_(d) is the voltage across the series connection 6 ofintermediate link capacitors, i_(ph) is the phase current and C, is thesnubber capacitance, i.e. the sum of total, series connected snubbercapacitance for one 2 of the valves, total, series connected snubbercapacitance for the other valve 3 and, whenever applicable, thecapacitance of the capacitor arranged between the phase output 10 andthe midpoint 9 of the intermediate link. If the auxiliary valve isturned on with a time delay corresponding to T/2 after the turn-off ofthe outgoing current valve, no current will pass through the auxiliaryvalve 18 and the inductor 17 during the commutation.

[0049] The changes of the phase voltage u_(ph) and the current i_(res)through the resonant circuit in connection with a commutation processwhere the auxiliary valve 18 is turned on at the same time as theoutgoing current valve is turned off (here denominated Case I), i.e. atthe instant t_(0l), are shown with continuous curves in FIG. 7. Thecorresponding changes in connection with a commutation process where theauxiliary valve 18 is turned on with a certain time delay t_(d) afterthe turn-off of the outgoing current valve (here denominated Case II),in which case the outgoing current valve is turned off at the instantt_(0ll) and the auxiliary valve is turned on at the instantt=t_(0ll)+t_(d), are shown with dashed curves. For illustrativepurposes, the curves have been drawn so that the zero passing of thephase voltage coincides in the two different cases.

[0050] As appears from FIG. 7, the current pulse through the resonantcircuit will get a shorter duration in Case II than in Case I.Furthermore, the peak value of the current pulse through the resonantcircuit will be lower in Case II than in Case I, as also appears fromFIG. 7. Consequently, a smaller charge amount will be supplied to orwithdrawn from the intermediate link midpoint in Case II than in Case I.It is realized that the magnitude of the difference insupplied/withdrawn charge amount between the two cases depends on themagnitude of the time delay t_(d). Whether a charge is supplied to orwithdrawn from the intermediate link midpoint 9 will of course depend onthe direction of the current through the resonant circuit, whichdirection in its turn is given by the direction of the phase current atthe commutation moment. It is realized that it is possible by a suitablechoice of time delay t_(d) to control how large amount of charge that issupplied to or withdrawn from the intermediate link midpoint 9 at aspecific commutation, whereby the voltage across the intermediate linkcapacitors 7, 8 and thereby the voltage unbalance in the intermediatelink can be adjusted in the desired direction.

[0051] The table below exemplifies how the time delay t_(d) generally isintended to be adjusted depending on the direction of the phase currentand the direction of the voltage unbalance in the intermediate link. Inthe table, Δu_(d) ^(f) denotes the filtered value ofΔu_(d)=u_(d1)—u_(d2). It is emphasized that the adjustment of the timedelay td is carried out also in consideration of other requirements thatare relevant for the commutation, such as maximum allowed duration ofthe commutation. i_(ph) > 0 i_(ph) < 0 Δu_(d) ^(f) > 0 t_(d) decreasedt_(d) increased Δu_(d) ^(f) < 0 t_(d) increased t_(d) decreased

[0052] The member 35 is preferably adapted to use the above-mentionedfiltered value of the voltage unbalance in the intermediate link in thedetermination of the time delay t_(d) in case a more advanced regulationof the unbalance is desired, previous values of the filtered value ofthe unbalance may also be used for determination of the time delay t_(d)in connection with a commutation. A PID-regulator (PID=ProportionalIntegral Derivative) may for instance be implemented for the purpose ofcorrecting the unbalance.

[0053] A noted unbalance of the voltage in the intermediate link isnormally adjusted in several steps, i.e. in connection with severalconsecutive commutations. The number of steps required for adjustment ofan unbalance depends i.a. on the magnitude of the unbalance and theupper limit for the time delay t_(d) for the present value of the phasecurrent. This upper limit depends i.a. on the requirements of a maximumallowed duration of the specific commutation.

[0054] If the converter has several phase legs and the resonant circuitsof the phase legs are connected to the midpoint of a common directvoltage intermediate link, the choice of the time delay t_(d) issuitably coordinated between the different phase legs. Since the phasecurrents normally sum up to zero, they will have different signs. Thiscreates good opportunities for counteracting unbalances in theintermediate link in accordance with the inventive method.

[0055] In previously known ARCP-converters, the control device is, inconnection with the commutation of the phase current from asemiconductor element of turn-off type of a first current valve to arectifying member of a second current valve, normally adapted to send aturn-on signal to the auxiliary valve only in those cases when the phasecurrent at the commutation moment has an absolute value being lower thana determined limit value G. This limit value is chosen based on the factthat the phase current, when the absolute value thereof at thecommutation moment is higher than the limit value, is to be capable ofrecharging the snubber capacitors without assistance of a resonancecurrent within a time that is short enough with respect to the controlof the converter. However, if the absolute value of the phase current islower than the limit value, assistance by a resonance current isrequired for allowing the recharging of the snubber capacitors to takeplace fast enough. Consequently, said limit value indicates how low thephase current is to be for the resonant circuit to be activated, i.e.for the auxiliary valve to be turned on during the commutation.

[0056] In the inventive converter and the inventive method, the resonantcircuit may, however, be activated for the purpose of correcting a notedvoltage unbalance in the intermediate link even when this is notrequired with regard to the commutation time, i.e. even when the phasecurrent is high enough for being capable of recharging the snubbercapacitors without assistance by a resonance current in a time that issufficiently short with respect to the control of the converter. In sucha case, the purpose of the activation of the resonant circuit isconsequently to correct the unbalance in the intermediate link and notto speed up the commutation process.

[0057] According to a special case of the inventive method, only thevalues t_(d)=0 and t_(d)=T/2 of the time delay td are used, i.e. valuesaccording to the two previously described extreme cases. This case isdistinguished from previously known solutions in that an adjustment ofthe above-mentioned limit value is used in the correction of adetermined voltage unbalance of the intermediate link. In this case, theconverter comprises means 38 for displacing the limit-value G_(+i) forpositive phase currents upwards or the limit value G_(−i) for negativephase currents downwards in dependence on the direction of the unbalanceso as thereby to correct the unbalance at least partly. Said means 38may be integrated in the control device 24, as illustrated in FIG. 6, orbe separate therefrom. The means 38 is preferably adapted to use theabove-mentioned filtered value of the unbalance in the determination ofthe magnitude and the direction of the displacement. The means 38 ispreferably adapted to adjust the magnitude of a limit value displacementin dependence on the magnitude of a determined unbalance.

[0058] A case where the limit value G_(+l) for positive phase currentshas been displaced upwards from the normal value G_(+i0) is illustratedin FIG. 8. It clearly appears from FIG. 8 that more commutations wherethe resonant circuit is assisting will take place when i_(ph)>0 thanwhen i_(ph)<0, whereby a larger charge amount than normally (with thepositive direction of the current defined by the arrow 39 in FIG. 1)will be withdrawn from the intermediate link midpoint 9 as seen over anentire network voltage period. In the corresponding manner, a largercharge amount than normally will be supplied to the intermediate linkmidpoint 9 as seen over an entire network voltage period when the limitvalue G_(−i) for negative phase currents is displaced downwards from thenormal value G_(i0). In this manner, an additional charge amount canconsequently be supplied to or withdrawn from the intermediate linkmidpoint 9, whereby the voltage across the intermediate link capacitorsand thereby the voltage in the intermediate link midpoint 9 can beadjusted in the desired direction.

[0059] In the table below, it is exemplified how the limit valuesG_(+i), G_(−l) generally are intended to be adjusted in dependence onthe direction of the phase current and the direction of the voltageunbalance in the intermediate link. In the table, Δu_(d) ^(f) denotesthe filtered value of Δu_(d)=u_(d1)−u_(d2). It is emphasized that theadjustment of the limit values G_(+i), G_(−i) is carried out also inconsideration of the other requirements that are relevant for thecommutation. i_(ph) > 0 i_(ph) < 0 Δu_(d) ^(f) > 0 G_(+i) displacedupwards G_(−i) displaced upwards Δu_(d) ^(f) < 0 G_(+i) displaceddownwards G_(−i) displaced downwards

[0060] It is realized that the turn-off and turn-on, respectively, ofthe semiconductor element of turn-off type of a current valve asdescribed above and as indicated in the claims, refer to thesimultaneous turn-off and turn-on, respectively, of all thesemi-conductor components 13 of turn-off type of a current valve inthose cases where the respective current valve comprises several seriesconnected circuits 12 of previously indicated type.

[0061] The invention is of course not in any way restricted to thepreferred embodiments described above, on the contrary manypossibilities to modifications thereof should be apparent to a personskilled in the art without departing from the basic idea of theinvention as defined in the appended claims.

1. A converter comprising: a series connection of at least two current valves arranged between two poles, a positive and a negative, of a direct voltage side of the converter, each of which current valves comprising a semiconductor element of turn-off type and a rectifying member connected in anti-parallel therewith, an alternating voltage phase line being connected to a midpoint, denominated phase output, of the series connection between two current valves while dividing the series connections into two equal parts, an intermediate link, arranged between the two poles of the direct voltage side of the converter, which intermediate link comprises a series connection of at least two intermediate link capacitors, said series connection being divided into two equal parts through a midpoint, denominated intermediate link midpoint, a resonant circuit comprising a series connection of an inductor and an auxiliary valve arranged between the phase output and the intermediate link midpoint, which auxiliary valve comprises at least two semiconductor components of turn-off type arranged in opposite polarity in relation to each other, the resonant circuit further comprising capacitive members, each of which being connected in series with said inductor and auxiliary valve and in parallel with one of said current valves, a control device for controlling turn-on and turn-off of the semiconductor elements of turn-off type of the current valves and the semiconductor components of turn-off type of the auxiliary valve, means for measuring the phase current and means for determining an unbalance of the voltage in the intermediate link, i.e. determining a difference between the voltage between one of the poles and the intermediate link midpoint and the voltage between the intermediate link midpoint and the other pole, wherein the control device is adapted, in connection with a commutation of the phase current from a semiconductor element of turn-off type of a first current valve to a rectifying member of a second current valve, to effectuate a turn-on of the auxiliary valve with a variable time delay after the turn-off of the first current valve, the converter comprising a member for determining said time delay in dependence on the magnitude and the direction of the phase current and the magnitude and the direction of a determined unbalance so that the unbalance is corrected at least partly when the control device applies the time delay.
 2. The converter according to claim 1, wherein the means for determining an unbalance of the voltage in the intermediate link comprises a device for filtering the value of said unbalance, the member for determining the time delay being adapted to determine the time delay in dependence on this filtered value.
 3. A converter according to claim 1, characterized in that the means for determining an unbalance of the voltage in the intermediate link comprises a device for measuring a first voltage between the intermediate link midpoint and one of the poles and a second voltage between the intermediate link midpoint and the other pole, and that this means is adapted to determine said unbalance by calculating the difference between said first and second voltage.
 4. The converter according to claim 1, wherein the converter comprises means for producing a displacement of a limit value, which controls for what values of the phase current the resonant circuit is to be activated in connection with a commutation, this means being adapted to displace the limit value for positive phase currents upwards or the limit value for negative phase currents downwards in dependence on the direction of the unbalance so as thereby to correct the unbalance at least partly.
 5. The converter according to claim 4, wherein the means for displacing the limit value is adapted to adjust the magnitude of a limit value displacement in dependence on the magnitude of a determined unbalance.
 6. The converter according to claim 1, wherein the respective current valve consists of several series connected circuits, each of which circuits comprising a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith, a snubber capacitor included in said capacitive members being connected in parallel with the semiconductor component of turn-off type included in the circuit.
 7. The converter according to claim 1, wherein the auxiliary valve comprises two semi-conductor components of turn-off type series connected in opposite polarity in relation to each other and two rectifying components connected in anti-parallel with a respective one of the semiconductor components of turn-off type of the auxiliary valve.
 8. The converter according to claim 1, wherein the auxiliary valve comprises several series connected sets of auxiliary valve circuits, each set comprising two series connected auxiliary valve circuits, each of which comprising a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith, the semiconductor components of turn-off type of the two auxiliary valve circuits in one and the same set being arranged in opposite polarity in relation to each other.
 9. A method for controlling a converter, which converter comprises a series connection of at least two current valves arranged between two poles, a positive and a negative, of a direct voltage side of the converter, each of which current valves comprising a semiconductor element of turn-off type and a rectifying member connected in anti-parallel therewith, an alternating voltage phase line (being connected to a midpoint, denominated phase output, of the series connection between two current valves while dividing the series connection into two equal parts, an intermediate link arranged between the two poles of the direct voltage side of the converter, which intermediate link comprises a series connection of at least two intermediate link capacitors, said series connection being divided into two equal parts through a midpoint, denominated intermediate link midpoint, a resonant circuit comprising a series connection of an inductor and an auxiliary valve arranged between the phase output and the intermediate link midpoint, which auxiliary valve comprises at least two semiconductor components of turn-off type arranged in opposite polarity in relation to each other, the resonant circuit further comprising capacitive members, each of which being connected in series with said inductor and auxiliary valve and in parallel with one of said current valves, a control device for controlling turn-on and turn-off of the semiconductor elements of turn-off type of the current valves and the semiconductor components of turn-off type of the auxiliary valve, means for measuring the phase current and means for determining an unbalance of the voltage in the intermediate link, i.e. determining a difference between the voltage between one of the poles and the intermediate link midpoint and the voltage between the immediate link midpoint and the other pole, wherein the control device, in connection with a commutation of the phase current from a semiconductor element of turn-off type of a first current valve to a rectifying member of a second current valve is made to effectuate a turn-on of the auxiliary valve with a variable time delay after the turn-off of the first current valve, said time delay being determined in dependence on the magnitude and the direction of the phase current and the magnitude and the direction of a determined unbalance so that the unbalance is corrected at least partly when the control device applies the time delay.
 10. The method according to claim 9, wherein the value of the unbalance of the voltage in the intermediate link is filtered, the time delay being determined in dependence on this filtered value.
 11. The method according to claim 9 wherein the voltage between the intermediate link midpoint and one of the poles and the voltage between the intermediate link midpoint and the other pole are measured, and that an unbalance of the voltage in the intermediate link is determined by calculating the difference, between said voltages.
 12. The method according to claim 9, wherein a displacement of a limit value is produced, which limit value controls for what values of the phase current the resonant circuit is to be activated in connection with a commutation, the limit value for positive phase currents being displaced upwards or the limit value for negative phase currents being displaced downwards in dependence on the direction of the unbalance, so as thereby to correct the unbalance at least partly.
 13. The method according to claim 12, wherein the magnitude of a limit value displacement is adjusted in dependence on the magnitude of a determined unbalance. 